Metamorphic Rock is formed when pre-existing rock deep within the Earth is altered by heat, pressure, and/or chemically active fluids. Metamorphic Rocks are classified into two groups: Foliated Metamorphic Rocks and Nonfoliated Metamorphic Rocks.
Foliated Metamorphic Rocks are rocks that form under contact metamorphisism, therefore they are more dense and have a banded appearance. Gneiss is a common Foliated rock.
Nonfoliated Metamorphic Rocks are rocks that normally only contain one mineral. Therefore, they have no banded appearance. Marble is a good example. | 
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